Process for preparing and modifying synthetic calcium carbonate

ABSTRACT

The present invention concerns a process for preparing calcium carbonate pigment from calcium oxide and/or calcium hydroxide and carbon dioxide in the presence of water. According to the invention, the starting materials are reacted in fluid state containing at least 20 parts of volume of gas for each part by volume of suspension formed by water and solid substances, and the amount of water is essentially equivalent to the amount which is evaporated during the reaction together with the amount left in a calcium carbonate product which behaves like a powder. By means of the present invention, PCC in powder form can be prepared without the product first having to be separated from a slurry.

This application is the national phase under 35 U.S.C. §371 of PCTInternational Application No. PCT/FI99/00033 which has an Internationalfiling date of Jan. 19, 1999, which designated the United States ofAmerica.

The present invention relates to a process according to the preamble ofclaim 1 for preparing synthetically produced pigment particlescontaining, in particular, calcium carbonate, whereby foreign substancesare added to the particles in connection with the forming of saidparticles in order to modify their properties.

According to a process of the present kind, a starting materialcontaining calcium oxide is reacted with carbonate ions and othermodification chemicals in the presence of water. Alternatively, theother modification chemicals are left unreacted among the rest of thematerial. The starting material may also comprise dry slaked Ca(OH)₂either together with unslaked lime or mixed therewith.

The use of calcium carbonate, particularly precipitated calciumcarbonate, is becoming increasingly common in many fields of industry,such as within the paper, the plastics and the pharmaceuticals industry.The aim is to formulate precipitated calcium carbonate (PCC) into afinely divided, pure pigment, the optical properties of which, e.g. thebrightness, are important properties for many applications. Syntheticcalcium carbonate (SCC) is a generic term covering also otherpreparation processes than traditional precipitation in liquid phase.

There are several known methods for preparing PCC. In our earlier FIPatent Application No. 950411 it is mentioned that very finely dividedPCC pigment can be prepared by using finely divided slaked lime as astarting material and by allowing the crystals grow essentially withoutmixing and by interrupting the reaction after a specific particle sizehas been reached by vigorous agitation.

In FI Patent Application No. 964132, said method has been furtherdeveloped by providing for monitoring of the viscosity of the nucleationmass in order to find out the proper point of time for interrupting thegrowth of the particles.

FI Patent Application No. 971161 discloses carbonation of calciumhydroxide with carbon dioxide in a mixing apparatus having high energydensity, said energy intensity being greater than 1000 kW/m³ in the freespace of the mixing zone of the apparatus.

Precipitation of calcium carbonate on the surface of foreign particlesis described in, e.g., published EP Patent Applications Nos. 0 375 683and 0 604 095.

By the above-mentioned processes, a PCC product in slurry form isobtained which has to be filtered if a completely dry product is to berecovered.

The present invention aims at providing a technical solution fordirectly producing PCC in powder form without the need of first havingto separate the product from a slurry, e.g. by filtration. The inventionalso aims at providing a process for easily modifying the properties ofthe PCC product in connection with the preparation process by combiningdesired modification chemicals therewith.

We have carried out tests aiming at preparing calcium carbonate which isas finely divided as possible. Surprisingly we have found in connectionwith these tests that the amount of water needed for ion formationduring the intermediate stage of the synthesis is very small incomparison to prior experiences and knowledge. Without restrictingourselves to any particular theory, it appears to us that the phenomenoncan be explained by the fact that even if a small amount of water iscapable of dissolving only an infinitely small amount of soluble ionsneeded for formation of said ions, the extremely large mass transferrate of the present process compensates for the water amount needed fornormal precipitation.

The present invention is based on the concept that successive processes,such as slaking of lime, i.e. calcium oxide, and carbonation of theslaked lime, are carried out in a high-energy apparatus in whichturbulence provided by the high energy intensity in the apparatusreplaces a slow process based on diffusion only in liquid and gas. Thereactions of the process are carried out at maximum dry matter content,in powder form, and as a result, the end product does not have to beconcentrated e.g. by filtering or by other methods but the end productis useful as such e.g. for the production of a slurry which can beemployed as filler or coating material of paper. Generally it can benoted that there is at least 20 parts by volume of gas in thecarbonation reaction for each part by volume of a suspension formed bywater and solid substances [primarily CaO, Ca(OH)₂ and CaCO₃)]. Inpractice, the water demand then only corresponds to the amountevaporated during the reaction (under the influence of the exothermalreaction and/or the processing temperature) together with the amountleft in the Ca carbonate product which behaves as a powder. Thus, whenthe temperature of the gas is, for example, about 100° C., only about0.8 to 1.2 parts by weight of water are needed for each part by weightof the Ca starting material. There will be left a maximum of about 40%of water in the product. In the process according to the invention,water (process water) is used as reaction water and for heattransfer/cooling.

According to the present invention calcium carbonate is not precipitatedon the surface of foreign particles in a continuous phase, as disclosedin published EP Patent Applications Nos. 0 375 683 and 0 604 095. Nor doearlier publications on PCC preparation suggest that carbonation couldbe initiated before the calcium oxide is slaked. According to thepresent invention the formation of the calcium carbonate takes placedirectly from calcium oxide or calcium hydroxide without intermediatestages in the form of a heterogeneous, three-phase synthesis. In thepresent context, “three-phase synthesis” means that during the formationof the calcium carbonate there is present a solid phase (calciumoxid/calcium hydroxide/calcium carbonate), a liquid phase (water andoptionally modifying agents dissolved in water) and a gas phase (carbondioxide). The calcium carbonate is formed in the liquid phase present onthe surface of a solid phase, and the calcium carbonate is released fromthe solid phase which forms its substrate of generation. The continuedgrowth of the released calcium carbonate crystals is stopped becausethey are not in contact with the reactant substrate longer. The releaseof the particles takes place under the influence of three differentfeatures: The strong growth of the solid phase during the reaction; thestrong temperature increase caused by the generation of reaction heat;and further the extremely strong turbulence in the apparatus.

Hydration of calcium oxide and carbonation of the hydratated part areperformed one immediately after the other under the influence ofefficient mixing. Then, according to the present invention, extremelysmall particles are at once formed which then immediately coalesce to 20nm primary particles which agglomerate to form strong 50 nm aggregateswhich further generate loose 100 nm secundary aggregates whichcorrespond to the balance between the forces acting on the particles.

These forces are, e.g., the capillary force caused by the water content,the van der Waals force, the mechanical forces caused by the turbulenceof the mixing and the electric forces caused by the Z-potential. Becausethe process by itself gives rise to a pH in the excellent range of about11, the isoelectric point of the Z-potential is close and there are nogreat resistance to the van der Waals forces. According to ourcalculations and measurements, a 1 to 5 molecules thick layer of wateris formed on the surfaces of the particles. All ion reactions andnon-ionic precipitations take place via said layer.

More specifically, the solution according to the present invention ismainly characterized by what is stated in the characterizing part ofclaim 1.

The present invention provides considerable advantages. Thus, theonce-through time of the process from raw materials to end product isonly on the order of some seconds. The present invention gives rise to amultifunctional process in which operations of earlier solutions arecombined to produce the desired end product with extremely shortresidence time and with a small operational content. Simultaneously, ithas become possible to remove intermediate depots and it has furthermorebeen found the the high-intensity mixing works best when the dry mattercontent of the treated materials is high with respect to water.

The behaviour of water in the process has been surprising. It is knownthat water in the form of steam is not capable of solubilizing salts,but we have learned that water spread out over a large surface in theform of a 1 to 5 molecules thick layer under the influence of thesurface properties (in the present case of the SCC), does not behave asa solvent, either. The phenomenon surprisingly provides an option ofinfluencing the crystal structure in a new way. This takes place when asuitable amount of water has been transferred to the gas phase byevaporation under the influence of a temperature increase.

The size of the forming particles can be regulated by adjusting the pHrange, e.g. with NaOH or H₂SO₄ and by changing the intensity of themixing and/or adjusting the initial amount of water. The startingmaterials of the process according to the invention comprise water,CaO/Ca(OH)₂ and CO₂. Of these substances, only calcium oxide and calciumhydroxide are actual variables. According to the invention it ispreferred to have the CaO ready slaked or to slake it in the process bymeans of so called dry slaking, under vigorous agitation so that theCa(OH)₂ structure become porous and the size of the formed particles is<3 microns and preferably <1 microns, when the agitation during theslaking is sufficiently intensive.

A particle produced by the above-defined three-phase heterogeneoussynthesis is opaque (i.e. it does not give any particular direction tolight) and its morphology is originally vaterite. This morphology isvery suitable for coating of paper, because high opacity can beobtained.

The calcium carbonate produced by the invention lends itself to use inparticular not only as a coating pigment of paper and cardboard but alsoas a filler of paper and cardboard. It can, however, also be employed asa filler and pigment for polymers, such as plastics and rubbers, andpaints and similar dispersions. The powdery product can be mixed withwater to form a mixture having a desired dry matter content of, forexample, about 60 to 80%.

In the following the invention will be examined more closely with theaid of a detailed description and with reference to the attacheddrawings.

FIG. 1 shows the principle of the generation of the product, thecarbonated particle being calcium oxide;

FIG. 2 shows the generation mechanism of the same product, calciumhydroxide particles being carbonated during the reaction; and

FIG. 3 depicts in a schematic fashion the basic structure of anapparatus according to a preferred embodiment of the present invention.

In the process according to the present invention, conventionalprecipitation is not used but the product is formed by means ofheterogeneous phase synthesis. The term SCC is also more descriptive forthis product because its properties differ essentially from those ofconventional PCC qualities as regards the fine structure of the basicparticles, their morphology and the very narrow size distribution of thebasic particles.

Modification of the homogeneous synthesized particles means that thesoluble matter originally dissolved in the accompanying water adheres tothe crystal lattice and/or amorphous matrix of the generated particle.The soluble matter does not form a separate phase of its own, but thereduced solubility of said matter caused by a reduction of theaccompanying free water forces these substances to transmigrate to thegenerated particles. The substances can be present in ionized form orthey can comprise other particles which are suspended in extra water, orsubstances which are emulgated in said water or colloidal macromolecularcompounds, such as proteins or carbohydrates. These ions are typicallyevenly distributed inside the carrier and partly on the surface thereof.Compounds of the present kind are e.g. Ba-, Mg-, Zn-, Al-, Mn-, Co- andCu-compounds in the forms of carbonates and other compounds formed bycations thereof and added anions of Si, Ti, S, P and F.

In the present context, inhomogeneous modification means that subphases,such as sulphate, silicate (e.g. water glass), sulphide, phosphate etc.,are formed in the carrier. These comprise separate islands which, inprinciple, can be separated from the crystalline or amorphous calciumcarbonate matrix.

The purpose of heterogeneous modification is to bring foreign materialsbetween the crystals or the amorphous matrix. These foreign materialschange the motion of light, they put obstacles to continuedcrystallization or they influence the chemical properties of calciumcarbonate, e.g. solubility or surface properties, which provide forenhanced affinity of the SCC particles towards, e.g. dispersants,retention aids, lubricants or colouring agents and which can be used(without limiting the applicability of the present patent) for furtherimproving whiteness, such as stilben derivative, and improvefluorescence properties, such as zinc sulphide (Cu activated) or to coatthe particle partly or entirely with coatings improving stabilityagainst dissolution such as phosphates or fluorides, or to improvedispersivity and decrease agglomeration, such as polyacrylates,polyacrylamides, starch and kationed starch. Some organometal compoundsform basic compounds which comprise alkoxy compounds which hydrolyse inwater and alcohol and which are very well suited to this purpose sincethey leave the surface of a SCC particle with a crosslinked coatingcomprising, e.g. titanium dioxide or silicon dioxide. There are manyother possiblities, but these cannot be discussed in detail in thepresent context. The aim has been to show that with the present processit is possible to implement an innumerable amount of additionaladditions or coating which cannot be obtained with other, known methodsas easily.

The PCC product obtained is in the form of a powder. By this is meant,in connection with the present invention, that it can be blown andseparated with a blowing test. The product does not contain free waterand its dry matter content is greater than 60%, preferably 80 to 100%.

The process according to the invention is divided into variousembodiments which all have in common the said quick throughput time ofthe process and the high-intensity mixing process needed for it.

Generally, quick lime and water are mixed in a fluid state in a powerfulmixer. In the fluid, the main part is gas that contains solid matter andliquid (dispersion+aerosol). The amount of gaseous phase in the fluid isat least 20 parts by volume per one part by volume of the suspension,and the amount of liquid phase is 1 to 20 parts per solid matter phase.The high-intensity mixing process preferably works in a fluid mediumwhere there are typically present, for example, 1000-10,000 parts byvolume of gas and steam/mist and 1 part of solid matter and 0.5 to 2parts of water, all indicated by volume. Treatment in such a fluid doesnot recognise viscosity limits or other phenomena brought about by it,such as difficult transfers of material in the intermediate phases. Morewater can be fed to the reaction but then due care has to be taken toensure evaporation or corresponding removal of the water, if a powderyproduct is to be obtained.

The process according to the invention is carried out via the stepsindicated by the following reactions:

CaO+H₂Oaq→Ca(OH)₂(+H₂O)

CO₂+H₂O→HCO₃ ⁻+CO₃ ²⁻

Ca(OH)₂+CO₂+CO₃ ²⁻→CaCO₃+H₂O

The dissolved modification agents and additives fed together with theslaking water of the lime are transferred into the formed particle andon the surfaces thereof. The mechanism for the generation ofnano-aggregates and agglomerates is also shown in the appended FIGS. 1and 2.

During carbonation, calcium oxide (FIG. 1) is thus subjected to anintensive agitation field together with water and carbon dioxide,whereby its surface layer begins to hydrate and, as a consequence of thehydration, Ca(OH)₂ is obtained which immediately, at the same time,begins to carbonate. The calcium carbonate obtained from the reaction isof even quality. Namely, very small PCC particles are generated in thecarbonation or causticising, correspondingly, onto the surface of thelime particles. As a consequence of the turbulence produced by themixing device, impact energy and the heat generated, these particles,however, detach from the surface of the calcium oxide or calciumhydroxide particles. They do not remain independent in the mixer fluidbut primary particles quickly combine to form larger particle aggregatesor clusters of about 10 to 30, typically about 15 to 20 particles. Theirsize is about 40 to 100 nm. The aggregates provide agglomerates, i.e.,botryoidal bunches that contain about 500-600 aggregates that combinewith one another. The size of the agglomerates is about 100 to 1000 nm,e.g. about 500 nm. They are fairly strong and endure the turbulence ofthe reactor. When larger, looser agglomerates are grown, the turbulenceis decreased. The formation of these agglomerates can be carried out byadjusting the pH value so that the Z-potential of the particles is aslow as possible.

The embodiment of FIG. 2 is analogous to that above described. Itcomprises, however, the difference that the starting material used iscalcium hydroxide instead of calcium oxide.

The particles size distribution of the product according to theinvention is rather steep, which is due to the precise control of theparticle formation. Thus, in practice, 90% or even 95% of the particlesare smaller than 500 nm.

As mentioned above, the properties of the produced PCC can be modifiedby feeding into the process additives which give the generated particlesthe desired properties, such as whiteness, opacity, fluorescense,phosphorescens, which improve the formation of triboelectric properties,increase electrical conductivity, reduce the solubility of the surfaceof the SCC particles at acid conditions, increase the adhesion of theparticles to anionic fibres or surfaces, or reduce the tendency of theparticles to agglomerate or flocculate. The modification chemical isdesignated the letter M in FIGS. 1 and 2.

In practice, the equipment according to the present invention functionsso that several high-power mixers/grinders are in series so that theyform a cascade in which, at least in the first stage, at least partialhydration of calcium oxide is carried out and, immediately after it orat the same time, the reagent, the carbon dioxide causing thecarbonation, is introduced. The calcium oxide can also be separatelyslaked and this feature is not a limiting characteristic of the presentinvention.

The whole process, from feeding the calcium oxide into the device andremoving the ready product from the device, takes 5 seconds maximum and0.1 seconds minimum, typically 1.5 to 3 seconds.

As mentioned above, the calcium carbonate particles generated in theprocess are not crystalline because normal crystallisation cannot takeplace in such a short onset time. They belong to the class of so-calledvaterite, i.e., amorphous calcium carbonate. This amorphousness and thecomplete round, spherical shape occurring at the same time, as well asvery precisely the same particle size distribution mean that the surfaceenergy of each discrete pellet is the same. Therefore, they are stabilein resisting crystallisation and dissolution and, further,crystallisation into a new shape that is thermodynamically more stabile.

The process can further be used for preparing structurized pigments,whereby another ready made pigment, such as kaolin, talc, chalk, PCC orTiO₂, is fed in addition to the starting raw materials, CaO and/orCa(OH)₂. Then, at least a part of the forming 20 nm basic particles areattached to the surface of ready carrier pigments and another partsforms aggregates of its own. Tribomechanical and triboelectrical points,to which the small SCC particles (20 nm) easily attach, are formed onthe coating particles due to impact and attrition. This structurizedpigment exhibits excellent opacity, in particular if the refractoryindex of the coated carrier particle is higher than that of the formedSCC particles. Both the difference between the refractory index and thegas-filled spaces between the particles provide for excellent opticalproperties. In addition, such spaces are excellent capillary adsorptionpoints for printing ink because they prevent lateral transfer of theink. This means rapid “drying” and sharp impressions.

The preferred coated particles are TiO₂ and Al₂O₃, aluminium oxide beingso much more inexpensive that it gives an economically better result. Ifthe basic particles in addition to CaCO₃ are further modified, e.g. suchthat they contain separate phases, this gives an optically moreadvantageous result also in the present case.

From all material technology it is generally known that amorphousmaterials tend to form crystalline phases as time passes, because theenergy level of crystals is lower than for amorphous masses. It isfurther known that the crystalline part present in amorphous matterchanges the motion of light (density, different refractory index). Incertain cases this feature may prove useful either optically or becauseof changes in the solubility of calcium carbonate particles; crystalsdissolving more slowly than amorphous matter. The degree ofcrystallization can be regulated by maturing (=time×temperature) orrestricted by impurities which, as known, generally comprise organicchemicals, typically sugars, glycols, polyglycols, polysaccharides,alcohols etc. dissolved in water. Both maturation of crystallinity andrestriction of crystallinity have their respective advantages as regardsthe intended further use. The process according to the present inventionmakes it advantageously possible to restrict the crystallinization byadding small amounts of above mentioned additives.

FIG. 3 shows a diagrammatic plan of an embodiment of the apparatus usedin the invention. The following reference numbers are used in FIG. 3:

1 Carbon dioxide container

2 Oxygen container

3 Propane container

4 Limestone feed

5 Storage funnel for limestone

6 Belt conveyor and weighing

7 Grinding of limestone

8 Combustion of limestone

9 Preheating of ground limestone and circulation gas

10 Cooling of quick lime and carbon dioxide

11 Equipment for treating the heat carrier

11 a Lifting elevator for the heat carrier

11 b Heat carrier sieve

11 c Temper screw for heat carrier

12 Heat exchanger

13 Slaking

14 Carbonation equipment

15 Stabilization; triboelectric charging apparatus for particles

16 Earthing and receiving tank for triboelectrically charged particles

17 Jet condensation system

18 PCC powder

19 Condensing water

20 Pretreatment of water

21 Circulation gas/carbon dioxide

In the schematic presentation of a production apparatus for PCC depictedin FIG. 3, the preparation is based on carbonation. The equipmentcomprises a part (reference numbers 4 to 7) where raw material, i.e.,limestone is mechanically treated, a burning unit for limestone(reference numbers 8 to 12), a carbonation unit (reference numbers 13 to16), and recovery and recycling of gases (reference numbers 17 and 21).Further, the equipment includes containers for the raw materials carbondioxide (reference number 1), oxygen (2) and propane (3).

The limestone crushed in storage funnel 5, fed along line 4, isoptionally preheated and, when needed, any snow and ice among thelimestone is melted. Belt conveyor transfers the limestone to beltconveyor scale 6. By adjusting the speed of the conveyor the amount oflimestone going into the process is adjusted. A metal detector isarranged in connection with the conveyor to detect possible metalobjects that are separated and transferred to a waste bucket.

Thereafter, the weighed amount of limestone is fed to grinding 7 wherethe limestone is ground by a two-step impact pulveriser, wherebylimestone powder is obtained, 90% of its particles having a size of lessthan 90 μm. The powder is conveyed from grinding 7 to preheaters 9, 10with the aid of a blower. Additional gas is brought to the suction faceof the blower from condensing jet 17.

The powdered limestone is preheated in a heat treating apparatus (heatexchangers 9, 10) the limestone being heated in the lower part 9 thereofand the burnt lime (calcium oxide) and the carbon dioxide being cooledin the upper part 10. In the preheater part 9, the hot (800-900° C.)heat transfer material flows down the middle channel of the heatexchanger and the fluidised limestone powder is blown through the bedthus formed in various phases by using the counterflow principle. Whenarriving at the heat exchanger, the temperature of the fluid is 20-100°C., increasing to about 700° C. in the heat exchanger. At the same time,the temperature of the heat transfer material drops to about 200° C.

Thereafter, the preheated limestone powder is conveyed to burning oflimestone 8 where the carbon dioxide is separated from the calciumcarbonate so that burnt lime, i.e., calcium oxide is produced accordingto the following equation: CaCO₃→CaO+CO₂. Burning is carried out influid tube 8 where the temperature of the particles is increased toabout 900-1400° C. by using burners. In the burners, propane is burnedwith oxygen, whereby carbon dioxide and aqueous steam is releasedthrough the reaction C₃H₈+5O₂→3CO₂+4H₂O. The propane is taken to burningfrom propane container 3 and oxygen from oxygen source 2 where it isseparated from air, e.g. by using a molecular sieve to produce pure O₂with a pressure of, for example, 2 bar.

The cold heat transfer material from the preheating section 9 oflimestone is circulated to the preheating equipment 9, 10 with anascending conveyor 11 a. The material obtained with the conveyor isscreened 11 b before it is retured via the temper screw 11 c to thecooling section 10.

1-5 mm crushed limestone can be used as the heat transfer material. Theburnt limestone powder obtained from the cooling section 10 is conductedto carbonation by using blower 12 and via a heat exchanger. The flowrate of the fluid in the cooling section 10 of the heat exchanger issimultaneously regulated with blower.

The calcium oxide is slaked, if desired, in slaking apparatus 13 towhich water is fed (cf. the embodiment of FIG. 2). After the optionalslaking, the raw material for carbonation is conducted to thecarbonation equipment 14. The equipment comprises several turbulencemixers of the impact pulveriser type which are arranged in series sothat they form a set of stages in a cascade. At each stage, the productat that stage can be modified. The process is essentially a parallelflow process where all the reactants move in the same direction. Thewater that determines the dry content of the product is fed to thedesired step of the carbonation equipment.

The product obtained from carbonation 14, i.e., the precipitated calciumcarbonate (PCC) is separated from the fluid gases (H₂O+CO₂).

The fluid gases of carbonation, i.e., water and carbon dioxide, arerecovered in jet condensing system 17 comprising condensing jet section,drop separator, and condenser. In the condensing jet section, gases arecooled with a water jet and the water vapour is condensed to water. Dropseparator prevents the water from ascending as drops to the upper partof the separator and condenser is used to cool the carbon dioxide thatgoes into circulation. Through pipe 21, the uncondensed gases recoveredin the condenser are returned to be used in the process and thecondensed water is removed from the bottom of condenser 19. The carbondioxide that is collected can be conducted , for example, through pipe22 to carbonation 14, to heat exchanger 9, 10 to be used as thecleansing blower gas of the heat transfer material and as the carriergas of limestone.

The desired product, i.e., the precipitated calcium carbonate (PCC), isrecovered as a dry PCC powder 18. If necessary, it can be slurried byfeeding water into receiver 16 from the apparatus 20 for pretreatment ofwater used, e.g., for removing ions from the water.

The operation of receiver 16 is the following:

The present process provides a very finely divided dry powder. Suchpowder causes, in principle, significant dusting and the recovery of thepowder from carrier gases has been considered a problem. Surprisingly,even said problem has been solved by the present invention by means of asimple solution. It has been found that if the SCC powder of the lastmixing stage is sufficiently dry (>95%), the particles aretriboelectrically charged under the influence of the mixing and they donot therefore agglomerate since they all have an electrical charge ofthe same sign. When the receiver 16, which may comprise a cyclone or asimple container, to which the dusty carrier gas is discharged, isearthed, the particles drift towards the walls of the vessel, theircharge is discharged and they are accumulated on the bottom of thevessel. In the test carried out, the effect has been so efficient thatthe jet washer 17 arranged after the receiver 16 used for cleaning gasfrom solid matter particles has only received about 0.5 to 2% of thetotal amount of dust for treatment.

In order to enhance said triboelectric effect during the last mixingstage, the product can be contacted with the surface of a ceramicmaterial. This can be carried out by, e.g., manufacturing the rotorwings from a ceramic material or by coating them with a surface layerconsisting of such a material.

Even if PCC even normally is present as a powder after the process, ithas been possible to raise the dry matter content by feeding additionalheat from outside the process into the circulating gas. Said heat hasbeen used for heating said circulating gas in order thereby to evaporatefurther water from the forming particles. The circulating gas of theprocess is then cooled after the separation of the particles in order toremove the extra water evaporated therein, and the water can berecirculated to the raw material fed into the process.

What is claimed is:
 1. A process for preparing a calcium carbonatepigment from calcium oxide and/or calcium hydroxide and carbon dioxidein the presence of water comprising reacting calcium oxide and/orcalcium hydroxide and carbon dioxide in a fluid state, wherein theamount of gas comprises at least 20 parts by volume for each part byvolume of a suspension formed by water and solids, and the amount ofwater comprises 0.8 to 1.2 parts by weight for each part by weight ofthe starting material CaO/Ca(OH)₂ when the temperature of the gas isabout 100° C.; and obtaining a powder carbonate product.
 2. A processfor preparing a calcium carbonate pigment comprising reacting calciumoxide and/or calcium hydroxide and carbon dioxide in the presence ofwater and further dissolving into the mixture of calcium oxide and/orcalcium hydroxide and carbon dioxide modification agents and additiveswhich are emulgated or suspended or colloidally dissolved therein, andwherein the water is in the form of steam or mist, and the volume of gasis at least 20 times larger than the volume of the mixture, and theformed powder calcium carbonate contains a small amount of water.
 3. Theprocess according to claim 1 or 2, wherein particles obtained from theprocess are separated from the gas phase by triboelectrical effect. 4.The process according to claim 3, wherein the process is carried out inseveral mixing stages, wherein the particles are dry at the last mixingstage so that the particles obtain an electrical charge which isutilized for separation of the particles from the fluid gas byconducting the particles to a grounded receiving device.
 5. The processaccording to claim 4, wherein the particles are contacted with ceramicmaterials during the last mixing stage.
 6. The process according toclaim 3, wherein external additional heat is brought to the circulatinggas for vaporizing more water from the formed particles by heating saidcirculating gas.
 7. The process according to claim 3, wherein thecirculating gas of the process is cooled after the separation of theparticles in order to remove superfluous evaporated water containedtherein.
 8. The process according to claim 1, according to which theproduction of the calcium carbonate takes place by a multistagecarbonization process, wherein additives and optional modificationagents and additives are added in the process at the earliest stageafter the first carbonization step, in order to provide a coatedheterogeneous additive zone.
 9. The process according to claim 3,comprising feeding into the process additives capable of giving theparticles the desired properties, such as whiteness, opacity,fluorescense, phosphorescense, or which are capable of improving theforming of triboelectric properties, of increasing electricalconductivity, of reducing solubility of the surface of the syntheticcalcium carbonate particles at acid conditions, of increasing theadhesivity of the particles to anionic fibers or surfaces, or ofreducing the tendency of the particles to agglomerate and flocculate.10. The process according to claim 1, wherein the product prepared is inionized form or the product is modified with an organic chemicaldissolved in water.
 11. The process according to claim 10, wherein theionized form is Si- or Ti-compounds which are organosilanes ororganotitanates which hydrolyze and precipitate on the surface ofsynthetic calcium carbonate particles functioning as carriers.
 12. Theprocess according to claim 1, wherein the carbonation reaction iscarried out in a fluid medium, in which there is present 100 to 10,000parts by volume of gas and steam, 1 part by volume of solid matter and0.5 to 1 parts by volume of water.
 13. The process of claim 10, whereinthe organic chemical is selected from the group consisting of glycols,polyglycols, polysaccharides and alcohols.
 14. The process of claim 10,wherein the ionized form is made of ionic compounds selected from Ba-,Mg-, Zn-, Al-, Mn-, Co-, Cu-compounds in the form of carbonates andother compounds formed by cations thereof and anions of Si, Ti, S, P andF.